In a thermal printing process, a dye bearing donor web is brought into contact with a dye receiving print media at a print zone. Thermal printing is effected by contacting the donor web with a print head that spans the donor web in a direction transverse to the direction of web travel. To maintain intimate contact between the donor web and receiver during the printing operation, the donor web and print media are partially wrapped around the surface of a print drum. The print drum is commonly driven by a precision stepper motor so that the spacing between adjacent image lines can be precisely controlled. Print quality is influenced considerably by longitudinal variations in the donor web tension during printing. Accurate color registration is not achievable when printing along the length of the web does not begin at the same point for each color frame. For good color registration, printing should begin at the same point on the receiver for each color frame so that each color frame exactly and precisely overlaps the previous color frame. To make a color print, color frames or patches of yellow, magenta and cyan are typically used and applied one on the other on the dye receiver. The color frames exist on the donor web in ordered fashion and are detected by a sensor.
During the color thermal printing process, it is necessary to have the dye donor web properly positioned relative to the dye receiver to ensure full coverage of the image area by successive color patches. This need has been met by placing color discriminating optical sensors directly in the donor path just past the thermal print head in the direction of travel of the donor web as illustrated in U.S. Pat. No. 4,710,781 which issued Dec. 1, 1987 to S. W. Stephenson. These sensors detect the presence of the different colored patches on the donor as they advance. When these sensors detect a new color frame during the printing cycle, the donor advance is stopped, properly positioning the donor relative to the receiver. It is desirable to position the sensors as close as possible to the thermal print line of the thermal print head because all donor that occupies that distance after positioning is not used and is therefore wasted. Unfortunately, the physical configuration of the print head and surrounding mechanisms limit the minimum distance that can be achieved. Accordingly, it will be appreciated that it would be highly desirable to have a thermal printer in which none of the dye donor is wasted by the inability to locate the sensor at the thermal print line.
Some thermal printers position the sensors at a specified distance upstream of the thermal head. As transitions in colors are sensed, the take up drive system advances the donor a predetermined distance to position the next color beneath the print line of the thermal print head. A problem with this positioning arrangement is that the precision of the positioning mechanism of the donor drive is affected by the diameter of the roll of accumulated expended donor which changes significantly from empty to full. Consequently, color frames at the end of the donor roll cannot be positioned as close to the print line as those at the beginning. Each frame must therefore be sufficiently large to accommodate this error, again causing valuable donor to be wasted. One solution to this problem is using encoders and software algorithms to measure the rotational speed of the supply spool while printing and to provide a compensated take up rate. Unfortunately, such a solution is expensive, and still has an unattractively high positioning tolerance. Accordingly, it will be appreciated that it would be highly desirable to precisely position the color frames without using encoders or other complicated or expensive components.